1. Field of the Invention
The present invention relates to an optical package substrate, an optical device, an optical module and a method for molding an optical package substrate. More particularly, the present invention relates to a package substrate having a surface configuration that is suitable for mounting optical components and/or optical elements thereon, and a method for molding the same, and relates also to an optical device and an optical module constructed by employing the package substrate.
2. Description of the Background Art
Optical communication systems employing optical fibers are evolving themselves from conventional long haul communication systems into subscriber communication systems. Subscriber-type optical communication systems require use of small and inexpensive optical devices and optical modules.
In conventional optical devices or optical modules, coaxial alignment has to be ensured between optical components, such as an optical fiber, a lens and an optical waveguide, or between an optical component and an optical element, such as a laser or a photo-diode. Usually, positioning of optical fibers, lenses, and optical waveguides requires high precision, e.g., a tolerance of ±1 μm. Therefore, a so-called “active alignment” method has been widely employed during an assembly process, in which positions of components are adjusted while driving an optical element to provide laser light and monitoring an amount of light that is being propagated through an optical fiber and optical waveguide. However, this technique requires complicated adjustment tasks, and is time-consuming, thus presenting a substantial cost problem.
On the other hand, so-called passive alignment, in which components are positioned without such adjustment has attracted attention as a technique for simplifying assembly of optical devices and optical modules. In one typical passive alignment technique, an optical module is assembled by using a silicon substrate, which can be wet-etched with high precision (Japanese Patent Laid-Open Publication No. 2001-21771, for example). This technique will now be described with reference to FIG. 26.
Referring to FIG. 26, an optical package substrate 261, which is composed of silicon, includes a guide groove (a so-called “V groove”) 262 for mounting an optical fiber 264, a marker 267 for positioning a laser 266, and a guide groove 263 for mounting a lens 265 (a flat micro-lens in the illustrated example) for converging laser light into the optical fiber 264. On the optical package substrate 261, the optical fiber 264 and the lens 265 are affixed to the guide grooves 262 and 263, respectively, and the laser 266 is fixed while being aligned with the marker 267, whereby these members are positioned with respect to one another.
With this conventional configuration, optical coupling efficiency is increased by use of the lens 265, while mounting of the optical fiber 264 and the lens 265 can be simplified. Note that although not shown in FIG. 26, a groove for holding an isolator may be formed between the guide groove 262 and the guide groove 263, whereby an amount of reflected return light to the laser 266 can be reduced.
However, the conventional optical package substrate 261 illustrated in FIG. 26 has the following problems.
The guide groove 262 for positioning an optical fiber can be formed by an anisotropic wet etching process of silicon. This process is based on a phenomenon that the (111) surface of silicon is selectively etched with a liquid etchant whose main component is KOH, and is capable of forming the guide groove 262 with a precise angle by using a patterned SiO2 mask.
However, because a side wall of the guide groove 263 for the lens 265 (or an isolator) extends vertically, but not in an oblique direction as does a V groove, the guide groove 263 is formed by a dicing process instead of a wet etching process. Particularly, since an optical axis of the lens 265 and that of the optical fiber 264 need to be aligned with each other, the guide groove 263 for the lens 265 needs to be formed with high precision on the order of microns with respect to its depth and position, thereby hindering mass-production thereof. Moreover, with a dicing process, the optical package substrate 261 needs to be processed across an entire width thereof in order to obtain a uniform groove depth (see the guide groove 263 in FIG. 26). Therefore, with the optical package substrate 261 illustrated in FIG. 26, it is necessary to actively adjust a position of the lens 265 in a direction perpendicular to the optical axis, i.e., a width direction of the optical package substrate 261. Moreover, the lens 265 that can be mounted on the substrate is limited to a flat lens.
As described above, in a case where a plurality of grooves of different cross-sectional shapes are formed in an optical package substrate made of silicon, it is necessary to perform different types of processes for these differently-shaped grooves.
Another method known in the art is to form a guide groove for fixing an optical fiber in a glass-based optical package substrate, instead of a silicon-based optical package substrate, by using press formation (Japanese Patent Laid-Open Publication No. 9-54222, for example). Note that press formation is a technique known in the art that has already been in practical use as a method for producing an aspheric glass lens. This technique will now be described with reference to FIG. 27.
First, a glass substrate 271 having a terrace (sunken portion) 272 in a center thereof, and a die 273 having a V-shaped protrusion for forming a groove (V groove) for guiding an optical fiber on one surface thereof, are provided ((a) in FIG. 27). Then, the die 273 is pressed against the glass substrate 271, which has been softened by high-temperature heating, thereby forming a guide groove 274 for an optical fiber in the glass substrate 271 ((b) in FIG. 27). During this process, the guide groove is not formed in the terrace 272 of the glass substrate 271. Then, an optical waveguide substrate 275 having an optical waveguide 276 therein is fitted into the terrace 272 of the glass substrate 271. Finally, an optical fiber 277 is affixed to each guide groove 274 ((c) in FIG. 27). Through steps as described above, the optical waveguide 276 and the optical fibers 277 are connected to each other.
Such press formation requires a die made of an ultra-hard alloy, or the like. Materials used in dies for plastic molding, such as electro-formed nickel, cannot be used because press formation of glass requires a die having both a high heat resistance and a high mechanical strength. FIG. 28A and FIG. 28B illustrate a typical, conventional method for forming a die used for molding a V groove. The die is formed by using a micro-grinder, which includes a disc-shaped diamond grindstone 281 whose tip is finished into a V shape, A die 283 as illustrated in FIG. 28B is obtained by grinding a flat ultra-hard alloy 282 from one end thereof with the grindstone 281, as illustrated in FIG. 28A. Then, produced die 283 is pressed against a glass material, which has been softened by heating, thereby transferring an inverted pattern of the die 283 onto the glass material. Thus, substrates having guide grooves (V grooves) formed therein can be mass-produced at low cost.
The method of pressing glass is better than the other method of etching silicon in terms of productivity. In addition, since glass, which is the material of the substrate, transmits UV light therethrough, a UV-curing resin can be used for mounting (fixing) an optical fiber on a package substrate. Therefore, it is possible to simplify and quicken a process of mounting optical components or optical elements.
However, use of a micro-grinder for producing a die for forming a V groove imposes limitations on a variety of die shapes that can be formed. For example, while a protrusion of a V shape can be formed with a disc-shaped grindstone, dies having other complicated shapes cannot be formed with a disc-shaped grindstone. Specifically, in order to obtain the optical package substrate 261 including the lens guide groove 263 near the guide groove 262 by press formation, as illustrated in FIG. 26, it is necessary to provide a die including a V-shaped protrusion for molding the guide groove 262 and a wall-shaped protrusion for molding the guide groove 263. However, when forming the V-shaped protrusion with a micro-grinder, the micro-grinder interferes with the wall-shaped protrusion, thereby preventing the die material from being processed in an intended manner. With a micro-grinder, it is very difficult to form a complicated shape as that in the illustrated example, and it is only possible to produce a die for forming a simple shape (e.g., a substrate with an array of parallel V grooves formed therein).
Thus, with conventional press formation for glass, it is not possible to produce a die having a complicated shape. Therefore, it is only possible to mold optical package substrates for limited applications (e.g., a substrate having V grooves along which optical fibers are placed for providing a connection to another component such as an optical waveguide). Due to such limitations, press formation using a die has not been employed for production of optical package substrates.
When forming the guide groove 274 for the optical fiber 277, as illustrated in FIG. 27, it is necessary to ensure a predetermined positional precision with the optical waveguide (i.e., the terrace 272 formed in the glass substrate 271) with respect to both a horizontal direction and a height (vertical) direction. Specifically, a tolerance of ±1 μm is required for alignment between the die 273 and the glass substrate 271 in the horizontal direction and also for a displacement amount by which the die 273 is pressed into the glass substrate 271 in the height direction, and this stringent tolerance is required every time a molding process is performed.
With the conventional method illustrated in FIG. 27, however, a variety of die shapes that can be formed with a micro-grinder is limited. Therefore, the terrace 272 in which an optical waveguide is positioned, and the guide groove 274 for the optical fiber 277 are formed separately. Specifically, in order to obtain, by press formation, the glass substrate 271 as illustrated in FIG. 27, it is necessary to provide a die including a V-shaped protrusion for molding the guide groove 274 and another protrusion for molding the terrace 272. However, when forming the V-shaped protrusion with a micro-grinder, the micro-grinder interferes with the protrusion for the terrace, thereby preventing the die material from being processed in an intended manner.
Thus, with an optical package substrate produced by a conventional method, it is difficult to ensure a high positional precision with a tolerance as stringent as ±1 μm, which is required for connection between a single-mode optical fiber and an optical waveguide. Therefore, a high production yield cannot be expected in mass production.